I suppose the counter argument would be how many times has Othello been played by a white actor, or indeed to see a film such as Lawrence of Arabia, where some of the Arabic warlords, Omar Sharif aside, were played by white men.I think casting for such roles is more authentic now, except there has been a reverse situation, where I think it can give a distorted version of the racial make up of Britain back in times when it was overwhelmingly white. I remember having a conversation with a black friend something along the lines of "there would have been a small black presence in Britain throughout history", her reply was "a lot were transitory and their numbers were miniscule in relation to the general population" Possibly more concentrated around dock areas, where Somali seamen for example settled, before the arrival of the Windrush generation.